The invention relates to a process and device for operating open baking furnaces, in particular open ring-type furnaces for manufacturing carbon-bearing, shaped bodies, especially electrodes for the Hall-Heroult fused salt electrolytic process for the production of aluminum.
The carbon-bearing bodies are usually produced from a mass of petroleum coke, anthracite, carbon black, graphite or the like mixed with a binding agent such as tar and/or pitch. On baking such bodies the binder is coked thereby producing the required mechanical and electrical properties.
This is the procedure used in particular with open ring-type furnaces to manufacture anodes for the aluminum fused salt electrolytic process.
The open baking furnace comprises a series of stationary baking chambers arranged in line next to each other. These chambers are separated from each other by headwalls running perpendicular to the direction of the series of chambers subdivided into pits and by flues running in the direction of the series of chambers.
By parallel arrangement of two rows of chambers and bridging over both flue systems, a ring-type of arrangement is obtained with the flues.
The charge, e.g. anodes for the aluminum fused salt electrolytic process, is introduced into the pits. In order to prevent fusing together and deformation of the anodes, and further to prevent the charge from burning away in the upper temperature range of the baking process an atmosphere which is as non-oxidizing as possible is provided by embedding the anodes completely in a packing powder of petroleum coke, metallurgical coke, anthracite or the like. This ensures that the anodes do not come in contact with each other, the flue walls or the floor.
The baking of the anodes takes place indirectly by heating the flues by means of external mobile burners. During the baking process, a plurality of chambers taken together form a so-called fire (German=Brandzug) in which the whole baking cycle takes place; these are connected up via a flue gas exhaust manifold to the flue ring main which as a rule circumvents the whole furnace. The number of chambers forming a fire depends on the geometry of the pits and flues and on the way the baking is carried out. The number of fires on the other hand depends on the size of the furnace i.e. on the number of chambers.
In general a fire comprises a sealing chamber, a pre-heating zone, a heating zone, each zone having about three chambers, and a cooling zone of about six chambers.
In the pre-heating zone the combustion gases are passed through the chambers filled with anodes which have not yet been baked, and then passed on to the flue ring main via the flue gas exhaust manifold.
Due to the flue gas exhaust manifold being connected to the flue ring main there is a negative pressure in the flues making up the fire. This causes the air required for the combustion of the fuel, generally gas or oil, to be drawn through the opened headwall openings behind the one to two chambers which are behind the heating zone and which are still charged with anodes which have already been baked--as a consequence of which that air is pre-heated. Also a considerable amount of air leaks into the flues, the air coming in through the porous walls, the closed burner openings and openings in the headwalls.
In the cooling zone, in order to cool the anodes after baking, air which to a certain extent is also used for combustion is introduced into the flues. This is done generally by means of two fresh air supply manifolds which are mounted on the headwalls either three and five, or three and four, or four and five chambers behind the heating zone. When compressive fans are employed the air escapes through the open burner openings and headwall openings in all chambers in the cooling zone.
The cooling capacity achieved depends greatly on the cross section of the openings through which the air can exit viz, headwall openings and burner openings. When using cooling fans, the excess pressure which may be employed is limited, as otherwise, especially in the vicinity of the fans, the packing powder and the charge will be subject to severe oxidation as a result of too strong a supply of air.
Situating the fresh air supply manifolds on neighboring headwalls e.g. three and four chambers behind the heating zone causes the cooling air to collect in the intermediate chamber as the air blown into the chamber by both manifolds can escape only through the small burner openings, whereas the air which is blown into the neighboring chambers on both sides can also escape through the free openings of the headwall.
There are also disadvantages associated with the arrangement of the fresh air supply manifolds behind the third and fifth chamber after the heating zone i.e. an arrangement with free headwall with free openings, situated between the manifolds. There is, apart from the burner openings, only one single headwall with free openings available for the cooling air to escape from the section between the manifolds; the cooling air from these two neighboring chambers can on the other hand escape through the openings of each appropriate headwall. In any case the chamber between the manifolds is not cooled adequately because there are insufficient openings for the air to escape.
The calcined anodes are removed from the chamber at the end of the cooling zone of a fire, and the empty chamber then reloaded with a charge of non-baked anodes.
By shifting the flue gas exhaust manifold, burners and fresh air supply manifold to a neighboring chamber at predetermined, regular intervals, the fire moves in a cyclic fashion (quasi-continuously) around the furnace.
According to the present state of the art the flue gas exhaust manifold is situated on the headwall, and the chamber immediately in front of the flue gas exhaust manifold functions as the sealing chamber, making it possible to create the necessary negative pressure in the fire in an controlled manner. For proper sealing it is necessary for the sealing chamber to contain charge and packing powder. Additionally the perpendicular opening in the headwall common to the sealing chamber and its immediately adjacent chamber where anodes are loaded or unloaded is sealed off with a sliding baffle. As a result of the negative pressure in the sealing chamber this baffle is pressed against a sealing surface. In order to increase the degree of sealing, the burner openings in the sealing chamber are additionally closed.
The productivity achieved with such a furnace depends on the rate of progress and number of fires and the capacity of the chambers.